Application Information

CIRCUIT DESCRIPTION

The LM2751 is a Switched Capacitor Convertor with gains of

2x and 1.5x. It is capable of continuously supplying up to

150mA at 4.5V or up to 80mA at 5V depending on the output

voltage option. The LM2751’s fixed frequency pre-regulation

maintains the output voltage to within 3% (typ.), making it

well suited for driving White LEDs. There are also four user

programmable switching frequencies to reduce the quies-

cent current consumption at light loads.

Aside from powering LEDs, the LM2751 is suitable for driv-

ing other devices with power requirements up to 150mA. The

LM2751 operates over the extended Li-Ion battery range

from 2.8V to 5.5V. The LM2751 limits output current to

250mA (typ.) during an output short circuit condition. LED

brightness is controlled by applying a PWM (Pulse Width

Modulation) signal to the Enable pin (EN). (see PWM

BRIGHTNESS CONTROL section).

SOFT START

Soft Start is engaged when the device is taken out of Shut-

down mode (EN = logic HIGH) or when voltage is supplied

simultaneously to the V

voltage on V

reference voltage is being ramped up. The output voltage is

programmed to rise from 0V to the regulated output voltage

level (4.5V or 5V) in 300µs (typ.).

ENABLE MODE

The Enable logic pin (EN) disables the part and reduces the

quiescent current to 0.77µA (typ.). The LM2751 has an

active-high enable pin (LOW = shut down, HIGH = operat-

ing) which can be driven with a low-voltage CMOS logic

signal (1.5V logic, 1.8V logic, etc). There is an internal 1M

Ω

pull-down resistor between the EN and GND pins of the

LM2751.

FREQUENCY MODE SELECT

The LM2751 switching frequency is user programmable via

two logic input pins, CS0 and CS1. Both logic input pins have

active-high logic (LOW = un-selected, HIGH = selected) and

can be driven with a low-voltage CMOS logic signal (1.5V

logic, 1.8V logic, etc). There are no internal pull-down or

pull-up resistors between the CSx and GND pins of the

LM2751. The CSO and CS1 can be controlled independently

or with the same logic signal.

The selectable switching frequencies are 9.5kHz, 37kHz,

300kHz, 725kHz. The switching frequency is programmed

according to Table 1

TABLE 1. Frequency Modes

CS0

CS1

Frequency

0

0

37kHz

0

1

300kHz

1

0

9.5kHz

1

1

725kHz

V

The LM2751 uses pre-regulation to regulate the output volt-

age to 4.5V or 5.0V depending on the voltage option. Pre-

regulation uses the voltage present at V

drive of the switched capacitor charge pump. This regulation

is done before the voltage is gained up by the charge pump,

giving rise to the term "pre-regulation". Pre-regulation helps

to reduce input current noise and large input current spikes

normally associated with switched capacitor charge pumps.

The LM2751 switched capacitor charge pump has gains of

2x and 1.5x. When the input voltage to the device is greater

than 3.58V (typ.), the LM2751 operates in a gain of 1.5x.

When the input voltage falls below 3.5V (typ.), the device

switches to a gain of 2x.

OUTPUT VOLTAGE RIPPLE

The primary contributor in keeping the output voltage ripple

of the LM2751 low is its switching topology. The output

capacitance, input voltage, switching frequency and output

current also play a significant part in determining the output

voltage ripple. Due to the complexity of the LM2751 opera-

tion, providing equations or models to approximate the mag-

nitude of the ripple cannot be easily accomplished. However,

the following general statements can be made.

The LM2751 has very low output ripple when compared to

typical boost regulators due to its double-pump topology,

where charge is continually supplied to the output during

both 2x and 1.5x modes. Combined with fixed frequency

operation modes, double-pumping allows for the use of a

very small, low value ceramic capacitor on the output node

while still achieving minimal output ripple. Increasing the

capacitance by adding a higher value capacitor or placing

multiple capacitors in parallel can further reduce the ripple

magnitude.

CAPACITOR SELECTION

The LM2751 requires 4 external capacitors for proper opera-

tion. Surface-mount multi-layer ceramic capacitors are rec-

ommended. These capacitors are small, inexpensive and

have very low equivalent series resistance (ESR,

≤15mΩ

typ.). Tantalum capacitors, OS-CON capacitors, and alumi-

num electrolytic capacitors are generally not recommended

for use with the LM2751 due to their high ESR, as compared

to ceramic capacitors.

For most applications, ceramic capacitors with X7R or X5R

temperature characteristic are preferred for use with the

LM2751. These capacitors have tight capacitance tolerance

85˚C), and typically have little voltage coefficient when com-

pared to other types of capacitors. However selecting a

capacitor with a voltage rating much higher than the voltage

it will be subjected to, will ensure that the capacitance will

stay closer to the capacitor’s nominal value. Capacitors with

Y5V or Z5U temperature characteristic are generally not

recommended for use with the LM2751. Capacitors with

these temperature characteristics typically have wide ca-

pacitance tolerance (+80%, −20%), vary significantly over

temperature (Y5V: +22%, −82% over −30˚C to +85˚C range;

Z5U: +22%, −56% over +10˚C to +85˚C range), and have

poor voltage coefficients. Under some conditions, a nominal

1µF Y5V or Z5U capacitor could have a capacitance of only

0.1µF. Such detrimental deviation is likely to cause Y5V and

Z5U capacitors to fail to meet the minimum capacitance

requirements of the LM2751.

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